This is an application to explore a novel approach to the in vivo analysis of antigen receptor function. Antigen receptor transgenic mice have been extraordinarily useful in the analysis of immune specificity, but they also pose a series of limitations to certain analyses. Taking advantage of these antigen receptor transgenic systems to screen the effects of various knockout mutations has been time consuming and cumbersome, primarily because of the need for introducing the transgenes onto the knockout genetic background. In this application we suggest a novel general strategy that we will apply to two particular questions in B-cell biology. The methods are however applicable to the analysis of several lymphocytic subsets and to the study of wide range of receptor-mediated processes. Specific Aim 1. To isolate single chain antibodies reactive to portions of antigen receptor chains. Specific Aim 2. To generate transgenic mice expressing these proteins as membrane antigens on the surface of selected tissue cells. Specific Aim 3. To demonstrate the feasibility of using the mice generated in Aim 2 to analyze antigen receptor-mediated processes in vivo. The long-term goal of this work is to generate a general approach to exploit the mutational analysis of receptor signalling. These studies should have broad application to question lymphocyte activation, differentiation, and survival.